Circuit for cutting-off output signal

ABSTRACT

An output signal cutting-off circuit includes a first switching element, a driving circuit and a voltage-drop-signal generating circuit. When a voltage to be monitored becomes lower than a threshold voltage, a voltage-drop-signal is generated and supplied to the driving circuit. The driving circuit turns on the first switching element based on the voltage-drop-signal to thereby cut off an output signal voltage by bringing it to a ground potential. When the voltage-drop-signal disappears, the first switching element is turned off to bring the output signal to a normal state. Preferably, a second switching element for charging a capacitor and a third switching element for discharging the capacitor are used in the driving circuit. In this case, the first switching element is turned on or off based on a voltage of the capacitor.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims benefit of priority ofJapanese Patent Application No. 2005-20177 filed on Jan. 27, 2005, thecontent of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electric circuit for cutting-off anoutput signal when a voltage to be monitored becomes lower than apredetermined level.

2. Description of Related Art

An example of a circuit for cutting-off an output signal is disclosed inJP-A-2003-304633. The circuit is for detecting an open-circuit failure.An essential portion of the circuit is shown in FIG. 5 attached hereto.A signal line 112 of a function circuit 101 is connected to a powersource line 141 through a resistor 121 and to a ground line 113 througha resistor 122. The power source line 111 is connected to the groundline 113 through a resistor 123.

The power source line 111 of the function circuit 101 is connected to apower source line 141 of a host circuit 104 that supplies power to thefunction circuit 101. The signal line 112 of the function circuit 101 isconnected to an output signal terminal 145 through a signal line 142 ofthe host circuit 104. The function circuit 101 includes a signalgenerating circuit 103. Signals from an amplifier 131 in the signalgenerating circuit 103 are outputted from the output signal terminal 145through the signal lines 112, 142. The ground line 113 of the functioncircuit 101 is connected to a ground line 143 of the host circuit 104.R01, R02 and R03 show internal resistances in the signal generatingcircuit 103.

In the host circuit 104, the power source line 141 is connected to thesignal line 142 through an internal resistance 124, and the signal line142 is connected to the ground line 143 through an internal resistance125. The power source line 141 and the ground line 143 of the hostcircuit 104 are connected to a power source terminal 144 and a groundterminal 146, respectively.

When the circuit for detecting an open-circuit failure described aboveis applied to a sensor for an automobile, such as a semi-conductorpressure sensor, a voltage of 5 volts is supplied to the functioncircuit 101 as a power source voltage VCC. When the circuit isfunctioning without an open-circuit failure, an output signal voltage ofthe function circuit 101 varies in a range of 0.3-4.8 volts. If thepower supply to the signal generating circuit 103 is discontinuedbecause of an open-circuit in the power source lines in the functioncircuit 101 and the host circuit 104, the output signal voltage Vout iscut-off by a pull-down resistance that connects the signal lines 112,142 to the ground lines 113, 143. The output signal voltage Vout becomesto a ground voltage level which is lower than an normal output signalvoltage. In this manner, the circuit proposed by JP-A-2003-304633prevents the output signal voltage from becoming unstable when theopen-circuit failure occurs.

In the proposed circuit, the output signal voltage Vout is cut-off byconnecting pull-down resistances to the signal lines 112 and 142.However, the following problems are involved in the proposed circuit.First, an output capacity of the amplifier 131 in the signal generatingcircuit 103 has to be increased because a current always flows throughthe pull-down resistances. This results in higher power consumption indetecting the open-circuit failure. Secondly, the output signal voltageVout may not decrease to the ground level immediately after anopen-circuit failure or a decrease in the power source voltage VCCoccurs. The delay is caused by charges stored in capacitors connected tothe signal lines 112, 142 and to the power source lines 111, 141, thoughthese capacitors are not shown in FIG. 5.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above-mentionedproblems, and an object of the present invention is to provide animproved circuit for cutting-off an output signal, in which the outputsignal is immediately cut off when a power source voltage decreaseswhile minimizing power consumption.

An output signal from a signal generating circuit is cut off, or broughtto a ground potential, by a circuit for cutting-off an output signalwhen a voltage to be monitored, such as a power source voltage, becomeslower than a threshold voltage. The cutting-off circuit is composed of:a voltage-drop-signal generating circuit that generates avoltage-drop-signal when the voltage to be monitored becomes lower thanthe threshold voltage, a first switching element, such as a MOS-FETtransistor; and a driving circuit for turning on or off the firstswitching element.

When the voltage-drop-signal is generated, the first switching elementis turned on by the driving circuit to thereby bring the output signalto a ground potential. In other words, the output signal is cut off whenthe voltage to be monitored becomes lower than the threshold voltage. Onthe other hand, when the voltage to be monitored recovers its normalvoltage, the first switching element is turned off to thereby terminatethe cutting-off operation.

The driving circuit may include a second switching element, a diode anda capacitor. In this case, the second switching element is turned onwhen the voltage-drop-signal is generated to charge the capacitorthrough the diode that allows current to flow only in the direction fromthe second switching element to the capacitor. The first switchingelement is turned on by a voltage of the charged capacitor. Further, athird switching element for quickly discharge the capacitor when thevoltage-drop-signal disappears may be used in the driving circuit. It ispreferable to use a field-effect transistor, a base of which isconnected to the capacitor, as the first switching element to maintainits on-state for a long period when necessary. The voltage to bemonitored may be a voltage supplied to the output cutting-off circuit asa power source voltage, or a voltage supplied to the signal generatingcircuit as an input signal for generating the output signal.

According to the present invention, the output signal is cut offimmediately when the voltage to be monitored becomes lower than thethreshold voltage, while minimizing power consumption in the outputsignal cutting-off circuit. Other objects and features of the presentinvention will become more readily apparent from a better understandingof the preferred embodiment described below with reference to thefollowing drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a circuit for cutting-off an output signalaccording to the present invention;

FIG. 2 is a diagram showing a circuit for generating avoltage-drop-signal used in the circuit shown in FIG. 1;

FIGS. 3A and 3B show alternative examples of a diode for preventingdischarge of a capacitor;

FIG. 4 shows graphs of various waveforms in the output signalcutting-off circuit; and

FIG. 5 is a diagram showing a conventional output signal cutting-offcircuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the present invention will be described withreference to FIGS. 1-4. A circuit for cutting-off an output signalaccording to the present invention is applied, for example, to a signalgenerating circuit in an acceleration detector or a pressure sensor foruse in an automotive vehicle. The circuit may be applied to other signaloutput circuits outputting signals obtained by calculations carried outtherein.

FIG. 1 shows an entire circuit including a circuit 4 for cutting-off anoutput signal (referred to as an output cutting-off circuit 4) accordingto the present invention. The output cutting-off circuit 4 is includedin a signal generating circuit 1. The signal generating circuit 1includes an amplifier 2 for amplifying a signal, such as a signal froman acceleration sensor of an automobile. A power source voltage VCC anda ground potential GND are supplied to the signal generating circuit 1from an electronic control unit 3 (referred to as an ECU 3) as a hostcircuit of the signal generating circuit 1.

An output signal Vout from the signal generating circuit 1 is fed to theECU 3. The ECU 3 includes a pull-down resistor 12 connected between theVout terminal and the GND terminal, so that the output signal voltageVout can be brought to the GND voltage level when any accident occurs inthe circuit. A resistance value of the pull-down resistor 12 is set to avery high value, such as 100 kΩ, so that an amount of current flowingthrough the pull-down resistor 12 is negligibly small in a normal state.

The output cutting-off circuit 4 included in the signal generatingcircuit 1 cuts off the output signal Vout from the signal generatingcircuit 1 when the power source voltage VCC becomes lower than athreshold voltage Vth. In other words, the output signal voltage Vout isbrought to the ground potential GND when the power source voltage VCCbecomes lower than the threshold voltage Vth. In order to detect whetherthe power source voltage becomes lower than the threshold voltage Vth, acircuit for generating a voltage-drop-signal Vlow (shown in FIG. 2) isincluded in the output cutting-off circuit 4.

As shown in FIG. 2, the voltage-drop-signal generating circuit 5 has apair of resistors 21, 22 for dividing the power source voltage VCC. Thedivided voltage is supplied to one terminal of a comparator 23. Thethreshold voltage Vth is fed to the other terminal of the comparator 23from a threshold voltage generating circuit 24. The threshold voltagegenerating circuit 24 is composed of a control transistor 25, a resistor26 and a Zener diode 27. The threshold voltage Vth is stabilized by aZener voltage of the Zener diode 27, i.e., the stabilized thresholdvoltage Vth is obtained when the power source voltage VCC decreases.

When the power source voltage VCC decreases, the divided voltagesupplied to one terminal (+) of the comparator 23 becomes lower than thethreshold voltage Vth supplied to the other terminal (−) of thecomparator 23. Accordingly, the voltage-drop-signal Vlow indicating thatthe power source voltage VCC becomes lower than the threshold voltageVth is outputted from the comparator 23. The voltage-drop-signal Vlow isa high level signal (a H-level signal). The level of thevoltage-drop-signal Vlow varies according to the power source voltageVCC as shown in the second graph in FIG. 4.

As shown in FIG. 1, an inverter 6, to which the voltage-drop-signal Vlowis fed, is connected to the voltage-drop-signal generating circuit 5.The inverter 6 outputs an inverted signal Vlow′ that is fed to bothgates of a P-channel MOS-FET (field-effect-transistor) 7 and anN-channel MOS-FET 11. When the inverted signal Vlow′ is L-level (lowlevel), the P-channel MOS-FET 7 is turned on while the N-channel MOS-FET11 is turned off. On the other hand, when the inverted signal Vlow′ isH-level, the P-channel MOS-FET 7 is turned off while the N-channelMOS-FET 11 is turned on.

A source terminal of the P-channel MOS-FET 7 is connected to the powersource voltage VCC, and its drain terminal is connected to a diode 8which is further connected to a capacitor 9. That is, the capacitor 9 ischarged with a diode voltage Vd supplied from the P-channel MOS-FET 7when a capacitor voltage Vc is lower than the diode voltage Vd. On theother hand, when the capacitor voltage Vc is higher than the diodevoltage Vd, discharge of the capacitor 9 is prevented by the diode 8. AP-channel MOS-FET 81 connected as shown in FIG. 3A may be used in placeof the diode 8. Alternatively, an N-channel MOS-FET 82 connected asshown in FIG. 3B may be used in place of the diode 8.

An N-channel MOS-FET 10 is connected between an output signal lineoutputting the output signal voltage Vout and the ground. That is, adrain terminal of the N-channel MOS-FET 10 is connected to the outputsignal line, and its source terminal is connected to the ground. Thecapacitor voltage Vc is supplied to a gate of the N-channel MOS-FET 10.The N-channel MOS-FET 10 is turned on when the capacitor voltage Vcexists, and thereby the output line potential is brought to the groundlevel potential, i.e., the output signal Vout is cut off. On the otherhand, when the capacitor voltage Vc does not exists, i.e. when thecapacitor 9 is discharged, the N-channel MOS-FET 10 is turned off.

A drain terminal of the N-channel MOS-FET 11 is connected to a pointbetween the diode 8 and the capacitor 9, and its source terminal isgrounded. The inverted signal Vlow′ is supplied to the gate of theN-channel MOS-FET 11. When the N-channel MOS-FET 11 is turned on, thecapacitor voltage Vc is immediately discharged through the N-channelMOS-FET 11.

The plural switching elements in the output cutting-off circuit 4 aremade of a CMOS circuit. Therefore, power consumption in the outputcutting-off circuit is suppressed. In particular, the switching circuitdriven by the capacitor voltage Vc is formed by a field-effecttransistor, i.e., the N-channel MOS-FET 10. Therefore, the capacitorvoltage Vc is not discharged through the N-channel MOS-FET 10, andaccordingly the N-channel MOS-FET 10 can be kept turned on for a longperiod of time.

Operation of the output cutting-off circuit 4 will be described belowwith reference to waveforms shown in FIG. 4. When the power sourcevoltage VCC becomes lower than the threshold voltage Vth, thevoltage-drop-signal Vlow at H-level is outputted from thevoltage-drop-signal generating circuit 5. The voltage-drop-signal Vlowis inverted by the inverter 6 to the inverted signal Vlow′ at L-level.This means that the Vlow′ signal at L-level is outputted when the powersource voltage VCC becomes lower than the threshold voltage Vth.

The P-channel MOS-FET 7 is turned on and the N-channel MOS-FET 11 isturned off by the inverted signal Vlow′ at L-level. The capacitor 9 ischarged with the power source voltage VCC through the P-channel MOS-FET7 and the diode 8, and thereby the capacitor voltage Vc appears at aterminal of the capacitor 9. The capacitor voltage Vc is supplied to thegate of the N-channel MOS-FET 10, and thereby the N-channel MOS-FET 10is turned on to bring the output signal voltage Vout to the groundlevel. Since a capacitor (not shown) for removing noises is connected tothe output signal line, the output singal Vout is dropped to the groundlevel with a slight delay, as shown in the bottom graph in FIG. 4. Sincedischarge of the capacitor voltage Vc is prevented by the diode 8, theN-channel MOS-FET 10 is kept turned on during a period in which thepower source voltage VCC is lower than the threshold voltage Vth. Thatis, the output signal voltage Vout is kept at the ground level duringthis period.

When the power source voltage VCC recovers and becomes higher than thethreshold voltage Vth, the voltage-drop-signal generator 5 outputs thevoltage-drop-signal Vlow at L-level which is inverted to the Vlow′signal at H-level. The P-channel MOS-FET 7 is turned off and theN-channel MOS-FET 11 is turned on by the Vlow′ signal at H-level. Thecapacitor voltage Vc is immediately discharged through the N-channelMOS-FET 11. As a result, the N-channel MOS-FET 10 is turned off torecover the output signal voltage Vout.

As explained above, the N-channel MOS-FET 10 is turned on by thevoltage-drop-signal that is generated when the power source voltage VCCbecomes lower than the threshold voltage Vth. The output signal Vout isimmediately brought to the ground level by turning on the N-channelMOS-FET 10, i.e., the output signal Vout is quickly cut off. When thepower source voltage VCC is normal (higher than the threshold voltageVth), the N-channel MOS-FET 10 is kept non-conductive. Accordingly, nocurrent flows through the N-channel MOS-FET 10, and power is notconsumed when the power source voltage VCC is normal. Further, since thecapacitor voltage Vc is quickly discharged when the power source voltageVCC becomes higher than the threshold voltage Vth, the cutting-off ofthe output signal Vout is immediately terminated.

The present invention is not limited to the embodiment described above,but it may be variously modified. For example, though the power sourcevoltage VCC is monitored in the embodiment described above, an inputvoltage supplied to the signal generating circuit 1 for generating theoutput signal Vout may be monitored. In this case, the output signalVout is cut off when the input voltage becomes lower than apredetermined level in the same manner as in the embodiment describedabove. However, if the VCC is stable, the diode 8 and the capacitor 9may be eliminated and the N-channel MOS-FET 10 may be turned on directlyby a voltage supplied from the P-channel MOS-FET 7. Though the switchingelements in the output cutting-off circuit 4 are constituted by CMOS inthe foregoing embodiment, it is possible to use bipolar transistors asthose switching elements.

While the present invention has been shown and described with referenceto the foregoing preferred embodiment, it will be apparent to thoseskilled in the art that changes in form and detail may be made thereinwithout departing from the scope of the invention as defined in theappended claims.

1. A circuit for cutting-off an output signal outputted from a signalgenerating circuit, comprising: a voltage-drop-signal generating circuitfor outputting a voltage-drop-signal when a voltage to be monitoredbecomes lower than a threshold voltage; a first switching element forgrounding the output signal; and a driving circuit for turning on thefirst switching element thereby to ground the output signal when thevoltage-drop-signal is generated, wherein the voltage to be monitored iseither a supply voltage, or a signal derived from the supply voltage. 2.The circuit for cutting-off an output signal as in claim 1, wherein thedriving circuit includes: a second switching element which is turned onwhen the voltage-drop-signal is generated; a capacitor which is chargedvia the second switching element when the second switching element isturned on; and a diode, connected between the second switching elementand the capacitor for preventing discharge of the capacitor, wherein thefirst switching element is turned on by a voltage of the capacitor. 3.The circuit for cutting-off an output signal as in claim 2, wherein atleast the first switching element is constituted by a field-effecttransistor.
 4. The circuit for cutting-off an output signal as in claim2, wherein the driving circuit further includes a third switchingelement which is turned on when the voltage-drop-signal disappears tothereby discharge the capacitor.
 5. The circuit for cutting-off anoutput signal as in claim 1, wherein the voltage to be monitored is avoltage supplied to the circuit for cutting-off an output as a powersource voltage.
 6. The circuit for cutting off an output signal as inclaim 1, wherein the voltage to be monitored is a voltage supplied tothe signal generating circuit for generating the output signal.
 7. Thecircuit for cutting off an output signal as in claim 1, wherein thethreshold voltage is greater than 0 volts.
 8. A circuit for cutting-offan output signal outputted from a signal generating circuit, comprising:a voltage-drop-signal generating circuit for outputting avoltage-drop-signal when a voltage to be monitored becomes lower than athreshold voltage; a first switching element for grounding the outputsignal; and a driving circuit for turning on the first switching elementthereby to ground the output signal when the voltage-drop-signal isgenerated, wherein the driving circuit includes: a second switchingelement which is turned on when the voltage-drop-signal is generated; acapacitor which is charged via the second switching element when thesecond switching element is turned on; and a diode, connected betweenthe second switching element and the capacitor for preventing dischargeof the capacitor, wherein the first switching element is turned on by avoltage of the capacitor.
 9. The circuit for cutting-off an outputsignal as in claim 8, wherein at least the first switching element isconstituted by a field-effect transistor.
 10. The circuit forcutting-off an output signal as in claim 8, wherein the driving circuitfurther includes a third switching element which is turned on when thevoltage-drop-signal disappears to thereby discharge the capacitor. 11.The circuit for cutting-off an output signal as in claim 8, wherein thevoltage to be monitored is a voltage supplied to the circuit forcutting-off an output as a power source voltage.
 12. The circuit forcutting off an output signal as in claim 8, wherein the voltage to bemonitored is a voltage supplied to the signal generating circuit forgenerating the output signal.
 13. The circuit for cutting off an outputsignal as in claim 8, wherein the threshold voltage is greater than 0volts.
 14. A circuit for cutting-off an output signal outputted from asignal generating circuit, comprising: a voltage-drop-signal generatingcircuit for outputting a voltage-drop-signal when a voltage to bemonitored becomes lower than a constant threshold voltage; a firstswitching element for grounding the output signal; and a driving circuitfor turning on the first switching element thereby to ground the outputsignal when the voltage-drop-signal is generated.
 15. The circuit forcutting-off an output signal as in claim 14, wherein the driving circuitincludes: a second switching element which is turned on when thevoltage-drop-signal is generated; a capacitor which is charged via thesecond switching element when the second switching element is turned on;and a diode, connected between the second switching element and thecapacitor for preventing discharge of the capacitor, wherein the firstswitching element is turned on by a voltage of the capacitor.
 16. Thecircuit for cutting-off an output signal as in claim 14, wherein atleast the first switching element is constituted by a field-effecttransistor.
 17. The circuit for cutting-off an output signal as in claim14, wherein the driving circuit further includes a third switchingelement which is turned on when the voltage-drop-signal disappears tothereby discharge the capacitor.